Retired Lt. Gen. Michael Flynn made an unreported trip to the Middle East in 2015 to work on a U.S.-Russian venture in Saudi Arabia before he joined the Trump campaign, possibly having multiple contacts with Saudi officials that he failed to disclose when seeking renewal of his security clearances, according to Democrats who are seeking detailed records of Flynn's travels.

"Most troubling of all, we have no record of Gen. Flynn identifying on his security clearance renewal application - or during his interview with security clearance investigators - even a single foreign government he had contact with," wrote Reps. Elijah Cummings and Eliot Engel, the ranking members of the House Oversight and Foreign Affairs committees, in a letter published on Monday.

The Democrats have demanded documents related to all of Flynn's work on the Saudi nuclear venture, which involved not only a Russian-U.S. effort to construct the nuclear reactors but also a plan to have Arab countries repay the Russians with the purchase of "Russian military hardware," the letter says, citing internal documents from companies involved in trying to solidify the deal.

The allegations in the letter appear to compound the legal troubles facing Flynn, President Trump's first national security adviser, a post that Flynn was forced to give up after reports surfaced alleging he had lied to Vice President Mike Pence about the nature of back-channel discussions he was holding with the Russian ambassador to the U.S., Sergey Kislyak.

Flynn's attorney told ABC News he would not comment on the new allegations. Neither X-Co Dynamics Inc./Iron Bridge Group nor ACU Strategic Partners, the companies reportedly involved in the venture, responded to requests for comment.

Cummings and Engel appeared to focus on evidence that they said suggested Flynn repeatedly misled investigators about his overseas work during the period after 2014, when he left as the head of the Defense Intelligence Agency and formed a consulting firm, the Flynn Intel Group. Flynn previously made a late acknowledgement in filings to the Department of Justice that immediately before joining the Trump White House, he was doing work that could benefit the government of Turkey.

This letter suggests that Flynn's previously undisclosed foreign work extended much further than previously revealed. During the summer of 2015, the letter says, Flynn traveled to the Middle East to pursue the nuclear venture - a trip he did not disclose.

He reported another trip, to Saudi Arabia, in October 2015 on security clearance forms, but the letter says he omitted key details of that trip, including who paid for it. He said he traveled with a "friend," whom he did not identify, had a speaking engagement that could not be confirmed by the agencies that booked his speeches and stayed at the King Khaled International Hotel, the letter says. Congressional investigators could find no evidence that such a hotel existed.

Lawmakers believe his travels were related to a proposed U.S.-Russian partnership that would have been financed by Saudi Arabia to build, operate and secure dozens of nuclear reactors throughout the Middle East, the details of which were outlined by internal company memos first published by Newsweek. Flynn was reportedly tasked with developing a security strategy for the network of reactors and calming the concerns of uneasy U.S. allies in the region, such as Israel and Egypt.

While proponents touted the potential security benefits of the arrangement, particularly the opportunity to secure radioactive waste that could be used to make nuclear bombs, there is evidence to suggest that U.S. entities might have been pursuing a more self-interested agenda. Designers of the venture, Newsweek reported, sought not only to jump-start the sputtering U.S. nuclear industry but also to drive a wedge between Russia and Iran, two powerful impediments to U.S. interests in the Middle East.

The U.S.-Russian partnership seemingly never materialized, but the letter notes that shortly after his trip, Saudi Arabia announced a $100 billion deal with Rosatom, Russia's state atomic energy corporation, to build 16 nuclear power plants.

"Gen. Flynn failed to disclose these contacts with Saudi or other foreign officials on his security clearance application or during his interview with security clearance investigators," wrote Cummings and Engel, "which could constitute further violations" of federal law.

ABC News' Ben Siegel contributed to this report.

Editor's note: This article has been updated to reflect that retired Lt. Gen. Michael Flynn made the Middle East trip at issue in 2015 before joining the Trump campaign as an adviser in 2016.